How are volcanoes formed?
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Volcanoes are formed when magma from within the Earth's upper mantle works its way to the surface. At the surface it erupts to form lava flows and ash deposits. Ash deposits result from more explosive activity and lava flows usually are associated with quieter activity. Anyway, once you have piled up lava and/or ash around the vent, you have a young volcano. After many many more eruptions you have a big volcano that is constructed of the ash and lava products of these eruptions.

Image:Volcano scheme.svg

Volcano key:
1. Large magma chamber
2. Bedrock
3. Conduit (pipe)
4. Base
5. Sill
6. Branch pipe
7. Layers of ash emitted by the volcano
8. Flank		 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15. Ash cloud